Routing protocol OSPF

OSPF States

Operation	State	Description
Establish neighbor adjacencies	1. Down state	Negotiate master / slave relationship and DBD packet sequence number. The master initiates the DBD packet exchange.
	2. Init state	Hello packets are received from the neighbor. They contain the sending router’s Router ID. Transition to Two-Way state.
	3. Two-Way state	On Ethernet links, elect a DR, and a BDR. Transition to ExStart state.
Synchronize OSPF Databases	4. ExStart state	Negotiate master / slave relationship and DBD packet sequence number. The master initiates the DBD packet exchange.
	5. Exchange state	Routers exchange DBD packets. If additional router information is required then transition to Loading; otherwise, transition to Full.
	6. Loading state	LSRs and LSUs are used to gain additional route information. Routes are processed using the SPF algorithm. Transition to the Full state.
	7. Full state	Routers have converged.

OSPF router configuration

r1(config)# router ospf ospf-process-id     !does not need to be unique on all routers
r1(config-router)# router-id x.x.x.x        !value in IPv4 format
r1(config-router)# network network-address wildcard-mask area area-id
r1(config-router)# network intf-ip-address 0.0.0.0       area area-id  !enabling advertisements via a quad 0 wildcard mask,
                                                                       !tells the router to enable interface Serial0/0/0 for the routing process
r1(config-router)# passive-interface intf
r1(config-router)# passive-interface default
r1(config-router)# auto-cost reference-bandwidth 100    !note bandwidth value here is Mb/s
r1(config-router)# ip ospf cost value                   !sets cost of the link

Adjusting the Interface Bandwidth

r1(config)# interface serial 0/0/1
r1(config-if)# bandwidth 64
r1(config-if)# end

Setting router-id

Option 1 via loopback interface

r1(config)# interface loopback 0
r1(config-if)# ip address 1.1.1.1 255.255.255.255    !creates a host route, a 32-bit host route does not get advertised as a route to other OSPF routers
r1(config-if)# end

Option 2 via router config

r1(config-router)# router-id x.x.x.x   !value in IPv4 format

Option 3 it will be the highest IPv4 active interface address in up state.

Differences between OSPFv2 vs OSPFv3

	OSPFv2	OSPFv3
Advertises	IPv4 networks	IPv6 prefixes
Source address	IPv4 source address	IPv6 link-local address
Destination address	neighbor IPv4 unicast address 224.0.0.5 all-OSPF-routers multicast address 244.0.0.6 DR/BDR multicats address	neighbor IPv6 link-local address FF02::5 all-OSPFv3-routers multicast address FF02::6 DR/BDR multicast address
Advertise networks	network router command	ipv6 ospf process-id area area-id interface configuration command
IP unicast routing	Enabled by default	ipv6 unicast-routing global configuration command
Authentication	plaintext authentication or MD5 authentication	IPv6 authentication, IPSec authentication

Troubleshooting

show ip protocols
clear ip ospf process   !This forces OSPF on R1 to transition to the Down and Init states. Required if router-id needs changing
show ip ospf neighbor   !State - The OSPF state of the interface. FULL state means that the router and its neighbor have identical OSPF LSDBs. 
                        !On multiaccess networks, such as Ethernet, two routers that are adjacent may have their states displayed as 2WAY. 
                        !The dash indicates that no DR or BDR is required because of the network type.
show ip ospf     !spf, backbone area, router-id
show ip ospf interface brief
show ip ospf interface serial 0/0/1   !cost, bandwidth

Default reference bandwidth for 100Mb/s is 10^8 100.000.000 therefore Fast Ethernet 100Mbps link cost is 1. Cost is calculated using formula:

cost = reference bandwidth bps / interface bandwidth in bps

show interface serial 0/0/1 | include BW    !verify the current bandwidth of an interface